Field of the Invention
The invention relates to a method for eliminating residual oxygen impurities from silicon wafers pulled from a crucible.
The use of zone-refined silicon for the production of active components is generally known. It has however been found for vertically structured power semiconductor components that the use of zone-refined silicon in mass fabrication is limited, since only wafers with a diameter .ltoreq.150 mm can be produced.
It is further known to use so-called epitaxial silicon wafers. These epitaxial silicon wafers are heavily doped silicon wafers to which a lightly doped epitaxial silicon layer is applied. However, the higher the threshold voltage for which the power semiconductor components to be processed are intended to be designed, the thicker the epitaxial layer must be. This in turn leads to high production costs.
Silicon wafers pulled from a crucible are also known. This is the so-called Czochralski process. It would make economic sense to use such crucible-pulled wafers, since wafers having very large diameters can be produced with the method. However, it has not to date been possible to use these Czochralski wafers in a number of applications, in particular for vertical power semiconductor components, since silicon wafers pulled from a crucible exhibit doping fluctuations (striations) and defects due to incorporated carbon and oxygen impurities, which impair the properties of the components.
The pulling of crystals from a melt using the Czochralski process is generally used for the production of single crystals. Using a suitably oriented seed crystal, which is briefly brought into contact with the molten surface and then pulled back up slowly, i.e. sometimes more slowly than 1 mm/min, it is possible to produce relatively large single crystals. The seed crystal may in this case be rotated, e.g. at 20 revolutions per minute, to ensure uniform crystallization as well as uniform incorporation of dopants provided by the melt.
It is important for the temperature profile at the boundary between the melt and the solid crystal to be vertically aligned, both in order for the growth to be free of mechanical stresses and for the doping to be homogeneous. If the surface of constant temperature is not planar, ring structures (striations) with microscopic doping fluctuations occur. Those doping fluctuations are a particular problem in the context of vertical power semiconductor components.
The choice of the crucible material is critical in the case of silicon. Available choices include quartz or graphite, graphite provided with a hard graphite surface layer (lustrous carbon), and boron nitride.
The high temperature of the melt, 1415.degree. C., means that impurities enter the melt from the crucible material.
The two main residual impurities of silicon single crystals pulled from a crucible are small amounts of oxygen and carbon (about 0.02 ppm). The carbon impurities which occur, originating from the crucible material, are not in general of critical importance since carbon does not have a doping action in silicon. The oxygen impurities, however, are a cause for concern.
The oxygen impurities in silicon pulled from a crucible have long been used as "intrinsic" gettering sites. The silicon wafers are thereby thermally cycled to produce a defect-free zone near the surface. The thermal cycle consists of a first high temperature step at about 1100.degree. C., followed by a low temperature step at about 650.degree. C., and a second high temperature step at about 1000.degree. C.
The thermal cycle, also referred to as a denuding process, is very strongly dependent on the original concentrations of oxygen and carbon in the silicon.
The first, high temperature step breaks up the oxygen deposits which are present, and thus makes it possible to diffuse the oxygen out from the surfaces of the silicon wafer. During the subsequent second, low temperature step, nuclei are produced in the bulk of the silicon wafer, i.e. below the denuded zone. Deposits grow at those nuclei during the subsequent high temperature step, and serve as gettering sites for oxygen, heavy metals and other defects during the production process.
The usable active zone in the case of this method, the so-called denuded zone, is only a few micrometers deep, thus rendering unsuitable the use of silicon wafer production, dealt with in this way, of active vertical power semiconductor components whose space charge zones extend about 100 micrometers or more into the bulk of the silicon wafer.